corrosion

corrosion §

• corrosion is a redox reaction

dry corrosion §

• direct reaction with oxygen in air to form metal oxide is known as dry corrosion.
• sodium is very reactive, it must be stored under oil to prevent contact with oxygen and corrosion.
• $4N{a}_{\text{ (s)}}+O_{2\text{ (g)}}\to 2N{a}_2{O}_{\text{ (s)}}$
• some dry corrosion, like with aluminium forms tough, impervious coating of aluminium oxide $A{l}_2{O}_3$, protecting metal from further contact with oxygen (this is called passivation).
  • iron on the other hand, is less reactive and when it does corrode, the coating it forms flakes, so it is unable to protect the iron underneath.
• things to note
  • active corrosion: porous oxide layers may form, and the corrosion will continue deep into metal.
  • sensitive to temperature: temp increases -> ROR increases
• 3 types
  • oxidative corrosion
  • liquid metal corrosion
  • corrosion by other gases
    • (so2, co2) gases react with exposed metal oxidising it.
      • remember hydrogen sulfide on silver

wet corrosion §

• electro-chemical
• 1. metal
• 2. gas
• liquid -> h2o
• Fe + O2 + H2O is common
• draw diagram lots of times to get it
• basic iron corrosion process
  • iron solid oxidises and h2o and o2 reduces to form iron hydroxide
    • this is not rust however, because this is not the colour of rust.
  • electrons are transferred through iron solid to second region where o2 is reduced in the presence of water to hydroxide ions
  • then forms fe(oh)3
    • further oxidised in the presence of o2 and h2o to produce iron(iii)hydroxide: red-brown precipitate
  • then forms fe2o3
    • flaky; porous rust, exposes iron underneath to further corrosion.
• rate of rust AFFECT
  • humidity => h2o in air UP => rate of rust UP
  • temp => rate of rust UP => freq of col UP, % of succ UP
  • prevention:
    • DOWN temp => prevents
    • in industry , materials corrode over time
      • DOWN temp in factory
      • contact process
      • SO2+O2 -><- SO3

corrosion prevention §

1 surface protection

• painting
  • preventing contact w/oxidants
  • drawbacks
    • paint fades/flakes away => x permanent
    • paint doesnt cover porous metal => incomplete

2 cathodic protection

• voltage based THINK OF THIS AS CATHODIC PROTECTION
  • definition: target metal (save): force it to become a cathode (site of reduction)
  • we force electrons into the site of reduction (attach negative side of battery)
  • the inert metal becomes anode
    • inert metal is submerged under usually water/soil (tons of metal ions) causing it to react
  • forced our target metal: cathode therefore no oxd can occur = no corrosion
  • drawbacks
    • expensive: constant electricity => permanent
    • maintenance:
      • anode: change soil
        • anode soil needs to be changed because the ions gets depleted n stuff 9?
      • cathode: scratch (?)
    • CONTEXT BASED: electricity => wont work without
  • benefits
    • permanent
    • green principles of chem
• utilising sacrificial anode (plating)
  • more R => metal reacts as an anode
    • preferentially/sacrificially oxidised
  • this forces our metal to be a cathode =D
  • drawbacks:
    • constantly renew: not permanent, more R metal has to be replaced
    • if target metal comes into contact with less R metal, then the target metal will be forced to oxidise. therefore, cant be used where metals/ions are present
  • benefits
    • cheap
    • whenever cathodic protection (voltage based) cannot be used 3 plating protection
• plate (coat)
  • inert: less reactive
    • less reactive coating
    • less R oxidises instead of target metal-> eventually forms oxide layer: forms protective layer
      • prevents contact between gas/o2
    • drawbacks (scratching problem)
      • why corrosion rate increases as scratches
        • oxidants can now contact Fe
        • fe will directly oxidise instead of cadmium: higher SOP (corrosion)
        • cd is in contact with o2, thus acting as a site of Red: cd increases surface area of the metal, e- released can flow through Fe and Cd
          • cause rate of ox increase, which increases rate of reaction.
    • 
  • galvanisation: more reactive
    • more R: higher SOP
    • this more R metal reacts instead of the metal you want.
    • this more reactive metal is called sacrificial anode.
    • galvanisation is electroplating of zinc onto target metal
    • coating target metal = more R metal.
    • drawbacks
      • only protects metal less R than zinc
      • not permanent: coat it again